I believe you can use your NT license provided that you keep the VM in place
and don't transfer it before 90 days to another machine. I don't know how
you plan on using it and I am not a legal expert. I am just going by what I
have read and what I have seen elsewhere. It might be one of those cases
where you need to actually read the license and even then it might be hard
to fathom. I only know that this is a significant issue and that VMware went
to bat on behalf of users and MS resisted. I know that Windows 7 has
Virtualisation if you pay for the right license. I also know that MS wants
to sell as many of those licenses as they can and aren't interested in
getting older versions to work for you.